1. Field of the Invention
The present invention relates to an internal EGR control system for an internal combustion engine, for controlling internal EGR in which combustion gases are caused to remain in cylinders.
2. Description of the Related Art
Conventionally, there has been proposed an internal EGR control system which is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-127180. This engine is provided with a variable intake valve timing mechanism and a variable exhaust valve timing mechanism both of a hydraulically-driven type, for changing the intake cam phase and the exhaust cam phase with respect to the crankshaft independently of each other. Further, the engine is also provided with an intake valve lift mechanism and an exhaust valve lift mechanism for changing the lift of intake valves and that of exhaust valves.
In the internal EGR control system, the valve overlap amount between each intake valve and the associated exhaust valve is controlled by controlling the above-mentioned variable valve timing mechanisms and the valve lift mechanisms, whereby the internal EGR amount is controlled. More specifically, according to the engine speed and load on the engine, a target valve overlap amount, a target intake cam phase, and a target intake valve lift are calculated, whereby the variable intake valve timing mechanism and the intake valve lift mechanism are controlled such that the target intake cam phase and the target intake valve lift are reached. Further, according to the target valve overlap amount and the actual intake cam phase, the target exhaust cam phase is calculated, and according to the target valve overlap amount and the actual intake valve lift, the target exhaust valve lift is calculated. Then, the variable exhaust valve timing mechanism and the exhaust valve lift mechanism are controlled such that the target exhaust cam phase and the target exhaust valve lift are reached.
However, since the variable exhaust valve timing mechanism is hydraulically driven, the response thereof is easy to vary depending on the operating conditions of the engine. For example, as the hydraulic pressure or oil temperature is lower, the response of the variable exhaust valve timing mechanism is lower, and hence, when the target exhaust cam phase is changed, it takes time before the actual exhaust cam phase converges to the target exhaust cam phase. In contrast, if the exhaust valve lift mechanism is electrically driven, the response thereof is more stable and higher than that of the variable exhaust valve timing mechanism. Therefore, when the target exhaust valve lift is changed, the actual exhaust valve lift converges to the target exhaust valve lift in a shorter time period.
Further, in this conventional internal EGR control system, the variable exhaust valve timing mechanism (target exhaust cam phase) and the exhaust valve lift mechanism (target exhaust valve lift) are both controlled (calculated) according to the target valve overlap amount. Therefore, in the case where the response of the variable exhaust valve timing mechanism is lower than that of the exhaust valve lift mechanism, even if the actual exhaust valve lift has converged to the target exhaust valve lift, the actual valve overlap amount deviates from the target valve overlap amount until the actual exhaust cam phase converges to the target exhaust cam phase, which results in lower control accuracy of the internal EGR amount. Inversely, in the case where the response of the variable exhaust valve timing mechanism is higher than that of the valve lift mechanism, even if the exhaust cam phase has converged to the target exhaust cam phase, the control accuracy of the internal EGR amount is also lower until the exhaust valve lift converges to the target exhaust valve lift.
Particularly, when the control of the internal EGR amount is performed not by valve overlap but by advancing the valve-closing timing of each exhaust valve so as to cause combustion gases to remain in the associated cylinder, the high-temperature internal EGR remains within the cylinder. Therefore, if the control accuracy is low, the combustion state varies, which can undesirably cause instability of combustion.